The invention is in the field of well logging and relates specifically to correlating the relative depth levels of different sets of well logging data derived from separate passes of investigating devices through boreholes or from the same pass of a device.
In well logging, measurements of various formation characteristics are taken by an investigating device which is lowered into a borehole at the end of a supporting cable extending from the surface of the Earth. Measurements are taken at specified intervals as the device is drawn up. Typically, the measurements are intended to provide indications of oil or gas bearing strata, and may be measurements such as resistivity, inductance, and the like.
It has recently become common to combine, through sophisticated data processing techniques, different sets of measurements taken during passes of investigating devices through the same borehole or through different boreholes in order to produce computed measurements of various characteristics of the investigated Earth formations. When combining sets of measurements in this manner, it is important that they be accurately correlated in depth, e.g., when combining two different measurements each indicated as taken at depth of 2000 feet, it is important to be certain that each of these measurements has been in fact taken at depth of 2000 feet.
The depth at which a measurement is taken is commonly indicated by a sheave-wheel device which is located at the surface of the Earth and provides a measurement of the length of cable that passes over the sheave-wheel and suspends the investigating device in the borehole. However, this device does not generally take into account cable stretch, although the cable may stretch differently for different passes of the investigating device and even for different portions of the same borehole, because of factors such as changes in the borehole size, possible size and shape differences between different measuring devices, and the like.
Because of the desirability of accurate depth correlation of different logs, efforts have been made in the past to accurately measure the actual depth of the investigating device. One prior art technique involves accurate instantaneous cable length measurements, and is disclosed in U.S. Pat. No. 3,497,958 granted to L. H. Gollwitzer on Mar. 3, 1970. The Gollwitzer system measures the tension of the cable at the surface of the Earth and at the investigating device and corrects the cable length measurements indicated by the sheave-wheel device for changes in stretch as reflected by the tension measurements. The Gollwitzer system also corrects for sheave-wheel calibration errors and for temperature effects on cable stretch. While the Gollwitzer system has been found to provide extremely accurate depth measurements in most cases, there are factors which can cause slight errors. For the usual situation, these errors are insignificant and can be ignored. However, in certain of the recently utilized highly sophisticated data processing techniques applied to combining logs, even certain slight errors can become significant.
Another prior art approach to taking cable stretch into account involves the use of data processing techniques to determine the depth match between two or more logs derived from separate passes of investigating devices through a borehole or even from the same pass of the device. This approach is disclosed in U.S. patent application of David H. Tinch, Bruce N. Carpenter and Elie S. Eliahou filed on Sept. 9, 1970 and assigned Ser. No. 70,709. It involves determining first assumption depth displacement values for a number of log depth levels through use of a suitable correlation function. The first assumption depth displacement values for a selected number of depth levels are then analyzed to determine more accurate depth displacement values. This approach works well, but the need still remains for techniques and means for carrying out depth correlation that start with conventional data, such as correlograms obtained from applying a correlation algorithm to a pair of logs, reduce the amount of data that must be processed, and reduce processing time and processing complexity.